Note: I am no multi gladiator, rank one or even considered to be one of the top shadow priests but I can give you an idea of how the class works and how I play it.I took Vanguard's enhance guide as an example and used his formula for my guide. I saw a post on the forum here the other day about helping the community out, so I wanted to make this guide for new SPs and old ones returning. This might not be the best guide and you might not even agree with some of the things I say but I tried my best.

Talents
For shadow priests there are really no set of talents that gives you the best possible choice. It all depends on who you're facing and who you're playing with. I like to experiment a lot when it comes to talents but I will try go give you a simple guide on how to choose yours.

Level 15:
Recommended: Psyfiend
This is the standard cookie cutter level 15 talent. The talent itself is poorly designed and can in some cases be extremely useful, and in other cases extremely wasted. I like to take it vs. most comps unless we face some melee cleaves like TSG and Kitty Cleave.

Recommended: Void Tendrils
This is recommended to take vs a lot of melee cleaves. It's a good and reliable ability to have vs. melee cleaves training your team. (Note that it DRs with other roots)

Level 30:
Recommended: Phantasm
In 5.1 this was a must have talent. But since the nerf in 5.2 priests have a more open choice when it comes to the level 30 talent. I personally go with Phantasm because the 5 seconds freedom can help land fears and etc.

Recommended: Angelic Feather
This is a useful talent when playing with a holy paladin. When he gives you freedom and you use this ability you gain a 60% speed increase for the duration. Extremely useful in some cases.

Level 45:
Recommended: Solace and Insanity
Commonly referred to as "Insanity" this new priest talent have proven itself to be extremely powerful vs. many teams. The raw damage output, often combined with Power Infusion, is enough to sometimes solo a healer in a blanket silence. Note that taking this requires you to be able to free cast for the duration. Maybe not the best choice vs. melee teams.

Recommended: From Darkness, Comes Light
You can't go wrong with this. Basically it gives you 40k+ Mind Spikes if you're lucky enough to have your Vampiric Touch providing you with a proc. Stacking up two procs, 3 orbs with dots on healer in a Deep Freeze or Silence is enough damage to force defensives from most healers if you're able to set it up right.

Alternative: Mindbender
Can be useful vs some comps like melee teams training you. Provides you with sufficient mana regeneration, and gives a nice burst when getting trained. I like to use this vs. KFC training me because I don't have much time free casting and using Mindbender when going for the kill is a good option.

Level 60:
Highly recommended: Spectral Guise
Spectral Guise makes you invisible for 6 seconds or until you're mirror image is hit three times. Can be extremely useful vs. casters and for landing fears on healers. (Note that putting a shield on yourself before using this can help you stay stealthed longer when having dots on you)

Recommended: Angelic Bulwark
Getting trained by melee cleaves is always tough. Maybe extra tough for SPs as they don't have an ability like blink or teleport. Angelic Bulwark is useful vs. melee cleaves in my opinion. Being trained hard makes Spectral Guise almost useless and they hit your mirror image so fast you instantly get out of stealth. Angelic Bulwark may help you survive in some cases.

Level 75:
Highly recommended: Divine Insight
You get a chance whenever your Shadow Word: Pain deals damage to get a free, instant Mind Blast. In for example 3v3 where you put up three Shadow Word: Pain the procs won't be rare.

Recommended: Power Infusion
Provides an extreme burst combined with Insanity. Requires you to be able to free cast and is not recommended to take without the Insanity talent.

Recommended: Halo
Can be extremely good in many cases. Setting up burst on a healer or healing a team mate inside a Smoke Bomb this ability can either save a life or take one. Standing at the max range (30 yards) with Trinket and CDs up it can heal for 200k and hit for 100k. Also good for killing Psyfiends etc.

Glyphs
As with the talents it all depends on what you're meeting.Prime Glyphs:
Must have:Dark Binding - Extremely useful and saves you a lot of global CDs.
Options:Fade - Nice vs. teams training you.Inner Fire - Good vs. melee teams training you.Mass Dispel - Really good for getting your healer out of CC and removing bubble and Ice Blocks. (Note this is changing is 5.3)Prayer of Mending - Gives your PoM a nice buff.Shadow Word: Death - Nice vs. mages to avoid getting Polymorphed.Dispersion - Can be useful.

The big question, Mastery vs. Crit?
This ultimately goes down to if you want to top the leaderboards on damage or actually kill your target. Top SPs go for crit. Some might still go for mastery but crit can help you kill targets. I am going to quote Talbadar (arguably the best SP) on this one:
"Crit provides more burst damage when going for kills than Mastery does. I heard Mastery doesn't give extra proc benefits and possibly no mana (still unsure about this). All I care about is the kill, not just extra damage."
I agree with Talbadar on this one, but after some testing on dummy I found out that the extra dot ticks gives you extra mana. But in the end mana isn't how you win a game.

Main Hand / Off-hand vs. staff:
Staff gives you more crit/spirit but Main Hand / Off-hand provides more Intellect so I recommend getting that.

Professions:
Blacksmithing is currently the best PvP profession, and teamed up with either Jewelcrafting or Enchanting they provide some nice buffs to your stats.

What is your PvP rotation?
I personally get my dots up rolling first. Then I try to get of Mind Blasts as soon as they're ready for these good Shadow Orbs and if I have nothing to do I spam dispel kill target and healer.
When I'm going for the kill I usually have 3 orbs before I dot up the target, (Halo), Mind Blast, three orb Devouring Plague and if I have Insanity I Mind Flay.

Macros and Keybindings
These are personal macros and keybindings as you may copy and use if you want to. Note you do not need these macros or keybindings to perform well, althought I can't play without them! I recommend having your own keybindings.
For party macros I hate using Party1 and Party2 and they always seem to change.
Party targeting:
I target my healer with CTRL-Mousewheeldown and DPS with CTRL-Mousewheelup

Thanks for reading through the post. I hope this helped you out if you needed help. If you have any other questions feel free to post in the comments and I will try to give a good answer. I'm sure other SPs will also contribute to providing answers.
If you feel I have failed miserably on this guide please give me a constructive reply and how I can change it to make it better.

"Crit provides more burst damage when going for kills than Mastery does. I heard Mastery doesn't give extra proc benefits and possibly no mana (still unsure about this). All I care about is the kill, not just extra damage."

This part is wrong. There were some beta builds where it didn't give mana and extra procs but it has been fixed long ago.

Thanks! Strange because I enabled Periodic Energy Gains in Interface and did some testing and when I saw the mastery proc i got two 6k mana gains :s

EDIT:
Did some testing again and I still got double mana back. Took a screenshot, but then again it might be bug but it seemed like i got double mana when looking at the mana bar.http://i.imgur.com/VsNxgqZ.jpg